x86/sev: Disable CPU hotplug while SNP is active

While SNP is active, every memory write is checked against the RMP to
protect SNP guest memory.  A core performs these RMP checks only once
SNP has been initialized via SNP_INIT and the SNP-enable bit in SYSCFG is
set on that core; the firmware requires the SNP-enable bit to be set on
every present CPU before SNP initialization.

A core that is not SNP-enabled and not SNP-initialized performs no RMP
checks at all, so there is no valid configuration with SNP active and any
CPU exempt from RMP checks.

The firmware determines which CPUs are present from the processor and the
BIOS/UEFI configuration (e.g. SMT disabled in the BIOS) and enumerates
them at SNP init; it is not aware of the OS bringing CPUs online or
offline afterwards.

SNP_INIT fails unless SnpEn is set on all CPUs, so a CPU that is offline
when SNP_INIT is issued, does not have SnpEn set, SNP_INIT fails, and
there can be no SNP guest memory.  OS CPU hotplug can thus diverge from
the firmware's expectations and break SNP.

Tie CPU hotplug to the SNP-enable bit: disable it in snp_prepare() before
SNP is enabled, and re-enable it in snp_shutdown() once the firmware has
disabled SNP.

If snp_prepare() fails before enabling SNP it re-enables hotplug itself;
once SNP is enabled hotplug stays disabled, including across a failed
SNP_INIT and across the legacy SNP_SHUTDOWN_EX path, both of which leave
SNP enabled.

A kexec target that boots with SNP already enabled, disables hotplug once
in snp_rmptable_init(), since snp_prepare() bails when SNP is already
enabled.

With CPU hotplug now disabled while SNP is active, the online CPU mask is
stable, so the cpus_read_lock() previously taken in snp_prepare() to
iterate it is redundant.  Drop cpus_read_lock()/cpus_read_unlock() here.

Suggested-by: Thomas Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://patch.msgid.link/cd6962a88d691a020c2bf40fe77a6a3f3852fdfc.1789594774.git.ashish.kalra@amd.com

0 files changed, 0 insertions, 0 deletions